The present invention includes methods and devices to apply predistortion to correct nonlinearities of a power amplifier in an OFDM symbol transmission system. More particularly, predistortion is patterned to take into account clipping of symbols and to match an effective input range of the predistorter with an average power output of the power amplifier. This invention may be applied to a variety of standards utilizing OFDM technology, including IEEE 802.11a, Hiperlan/2 and MMAC.
A major challenge in OFDM transceiver design is ensuring the linearity of the transmitter and particularly the RF power amplifier (PA). The PA has a maximum output amplitude that is determined in part by the power supply and by the PA efficiency. As the input levels become large, the PA approaches saturation and the output amplitude and phase no longer display a linear relationship with the input. This non-linearity causes spectral regrowth due to intermodulation products and loss in transmit modulation accuracy. These effects are particularly evident in systems compliant with the wireless networking standard 802.11a, where the transmit signal has a large dynamic range and large constellations are used requiring greater transmit signal fidelity.
The typical approaches to PA non-linearity are to either increase the power backoff (i.e. decreasing the input range to the PA such that the PA operates in its more linear region) at the cost of output power and PA efficiency or to use a more linear and consequently more expensive PA. Less typical approaches include LFNC transmitters and analog feedback compensation circuits. A LINC transmitter consists of two parallel transmitters each transmitting a fixed magnitude signal. By adjusting the phase between the two signals and summing them, any variable magnitude signal can be generated.
An alternate method of avoiding PA non-linearities is to reduce the dynamic range of the transmit signal using peak to average ratio (PAR) reduction techniques. Unfortunately, due to the design constraints imposed by 802.11a compliance, the performance of PAR reduction techniques alone is not anticipated to be very significant.
Accordingly, an opportunity arises to develop an OFDM predistortion method and apparatus that improves system performance.